Pneumonectomy (PNX) leads to chronic asymmetric ventilatory loading of respiratory muscles (RM). We measured RM energy requirements during exercise from RM blood flow ((Q) over dot) using a-fluorescent microsphere technique in dogs that had undergone right PNX as adults (adult R-PNX) or as puppies (puppy R-PNX), compared with dogs subjected to right thoracotomy without PNX as puppies (Sham) and to left PNX as adults (adult L-PNX). Ventilatory work ((W) over dot) was measured during exercise. RM weight was determined post mortem. After adult and puppy R-PNX, the right hemidiaphragm becomes grossly distorted, but (W) over dot and right costal muscle mass increased only after adult R-PNX. After adult L-PNX, the diaphragm was undistorted; (W) over dot and left hemidiaphragm RM (Q) over dot were elevated, but muscle mass did not increase. Mass of parasternal muscle did not increase after adult R-PNX, despite increased (Q) over dot. Thus muscle mass increased only in response t!